It is known that at the time a positive electrode active material layer is formed by coating and drying a slurry (hereinafter, referred to as “positive electrode slurry”) containing a solution prepared by dispersing a positive electrode material and a binder in an organic solvent, and kneading and mixing the solution thereof, an organic acid is added to the organic solvent to prevent gelation during the preparation of the positive electrode slurry (see, Patent Literature 1).